1. Field
The present disclosure relates to a capacitive pressure sensor including a sensor chip having a diaphragm structure that detects a capacitance corresponding to pressure of a medium to be measured.
2. Description of the Related Art
In pressure sensors, including vacuum gauges which are used for example in semiconductor manufacturing facilities, a sensor element including a small diaphragm has been often adopted using a so-called micro-electromechanical systems (MEMS) technique. A main detection principle of this sensor element is that the pre sure of a pressure medium is received by the diaphragm and the resulting displacement of the diaphragm is converted into some type of signal.
For example, as a pressure sensor that uses a sensor element of this type, a capacitive pressure sensor is widely known. The capacitive pressure sensor is configured to detect, as a change in capacitance, the displacement of a diaphragm that flexes in response to pressure of a medium to be measured (hereinafter referred to as “measured medium”). Unlike Pirani gauges and ionization gauges, the capacitive pressure sensor is independent of the type of gas, resistant to corrosive process gases, capable of reducing adsorption of raw material gasses and accumulation of byproducts by heating the sensor element, and thus is often used, for example, in semiconductor facilities and various industrial applications. The diaphragm that flexes in response to pressure of the measured medium is called a pressure-sensitive diaphragm or sensor diaphragm.
For example, a capacitive pressure sensor that is configured and used to measure a vacuum in a manufacturing process in a semiconductor manufacturing apparatus is referred to as a diaphragm gauge. Examples of applications using the diaphragm gauge include film deposition which is performed by sputtering, chemical vapor deposition (CVD), or atomic layer deposition (ALD). The diaphragm gauge is also used in the process of etching, for example, silicon (Si) wafers.
In the film deposition or etching process, deposited films or byproducts produced during the process (hereinafter, these substances are referred to as “contaminants”) accumulate more or less inside the chambers, pipes, and pumps and cause various problems. Accumulation of contaminants inside the diaphragm gauge that measures and controls gas pressure in the process is known to cause a shift in zero point, change sensitivity to pressure, and significantly affect the quality of film deposition or etching.
Aside from keeping the sensor element at high temperatures, some measures have been taken to prevent accumulation of contaminants inside the diaphragm gauge. For example, Japanese Unexamined Patent Application Publication No. 2015-34786 discloses a method in which a path through which a process gas travels to reach the surface of a sensor diaphragm is made complex so that a gas which tends to adhere is caught along the way as much as possible. Also, Japanese Unexamined Patent Application Publication No. 2015-184064 discloses a method in which even when contaminants accumulate on a sensor diaphragm, the displacement of the diaphragm is reduced by controlling the position of the contaminants or modifying the structure of the diaphragm. Particularly for a film deposition technique using a surface reaction, such as ALD, a baffle structure is effective, which causes gas molecules to more frequently collide with the wall of a gas introduction path of a diaphragm gauge.
However, typical CVD is plot necessarily performed under the same conditions as ALD for the diaphragm gauge. In particular, because of recent miniaturization of semiconductors, a process which involves etching in the middle of film deposition has begun to be performed, instead of a simple CVD process. In the new process, where etching is performed with an etching gas different from a gas for film deposition, re-adhesion (accumulation) of contaminants onto the sensor diaphragm associated with chemical reactions between different substances and the resulting reaction heat may cause the diaphragm gauge to malfunction. The diaphragm gauge needs to be resistant to such processes, and improvement is required.
This challenge will be specifically described with reference to FIG. 14. FIG. 14 illustrates a configuration of a main part of a diaphragm gauge according to the related art. A diaphragm gauge 100 (100A) includes a diaphragm unit including a diaphragm (sensor diaphragm) 101 displaced in response to pressure of a measured medium and a diaphragm support portion 102 configured to support a periphery of the sensor diaphragm 101, a sensor base 105 joined to one side of the diaphragm support portion 102 and configured to define a reference vacuum chamber 104 together with the sensor diaphragm 101, and a base plate 107 joined to the other side of the diaphragm support portion 102 opposite the Sensor base 105 and configured to define a pressure introducing chamber 106 together with the sensor diaphragm 101.
In the diaphragm gauge 100A, a fixed electrode 108 is formed on a surface of the sensor base 105 adjacent to the reference vacuum chamber 104, and a movable electrode 109 is formed on a surface of the sensor diaphragm 101 adjacent to the reference vacuum chamber 104 in such a manner as to face the fixed electrode 108. The base plate 107 has a pressure introducing hole 107a in the center thereof (corresponding to the center of the sensor diaphragm 101). In the diaphragm gauge 100A, the measured medium is introduced through the pressure introducing hole 107a into the pressure introducing chamber 106 and causes the sensor diaphragm 101 to flex.
In the diaphragm gauge 100A, where the pressure introducing hole 107a is formed in the center of the base plate 107, a contaminant accumulates on a pressure receiving surface 101a of the sensor diaphragm 101 located directly below the pressure introducing hole 107a. That is, in the case of a gas phase reaction chemical reaction caused by collisions between molecules in the space), as illustrated in FIG. 15, a contaminant 110 accumulates in the center of the sensor diaphragm 101 as different substances chemically react with each other. This is because, as described below, a flow of the measured medium in the space above the pressure receiving surface 101a of the sensor diaphragm 101 facing the pressure introducing hole 107a not a molecular flow. Thus, in the diaphragm gauge 100A having this structure, the accumulation of the contaminant 110 in the center of the sensor diaphragm 101 causes a significant shift in zero point.
In a diaphragm gauge 100 (100B) illustrated in FIG. 16, the base plate 107 has, for example, four pressure introducing holes 107a outside the center thereof (corresponding to the center of the sensor diaphragm 101). That is, the base plate 107 has the four pressure introducing holes 107a in or around the area between a pressure sensitive capacitance and a reference capacitance. The four pressure introducing holes 107a are circumferentially evenly spaced apart from one another. As illustrated in FIG. 17, in the case of a gas phase reaction a chemical reaction between different substances causes the contaminant 110 to accumulate on the pressure receiving surface 101a of the sensor diaphragm 101 located directly below the pressure introducing holes 107a circumferentially evenly spaced apart from one another. Again, this is because the flow of the measured medium in the space above the pressure receiving surface 101a of the sensor diaphragm 101 facing the pressure introducing holes 107a is not a molecular flow. Displacement of the sensor diaphragm 101 caused by accumulation of the contaminant 110 is reduced in this case, but the accumulation of the contaminant 110 still causes a shift in zero point.